This invention relates generally to methods and apparatus for efficiently utilizing data acquisition hardware in CT scanners, and more particularly to methods and apparatus for adjusting data acquisition hardware in CT scanners to efficiently accommodate objects of different sizes.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
At least one known CT imaging system is a multiple-slice CT scanner that scans over a large area of an imaged object in a short period of time. In comparison with single-slice CT scanners, multiple-slice CT scanners have an x-ray source projecting a more widely collimated x-ray beam and multiple rows of detectors. This scanner is thus able to reconstruct thinner slices of images (i.e., the scanner has finer spatial resolution in the table translation, or z-direction) when scanning a patient, and does not require x-ray tube cooling.
To acquire multiple rows of data from the detector of the multiple-slice CT scanner, however, requires an increased amount of data acquisition hardware. For example, if there are m rows of n detector cells, there must be mxn data acquisition channels. By comparison, a single-slice CT scanner has only n detector cells and only n data acquisition channels.
One multiple-slice CT scanner having 16 rows of detector channels is constructed in such a manner that 16 rows of 1.25 mm slices of data are made available for image reconstruction. However, to utilize all of these rows requires both a large communication bandwidth in sending data through a slip ring to the host computer, and a large amount of data acquisition hardware to acquire all of the channels. In one known CT scanning system, only 4 rows of data are acquired simultaneously. Therefore, this system provides four different slice thicknesses, and only four slices at one time, specifically, 4.times.1.25 mm (central four detector rows), 4.times.2.50 mm (central 8 rows, adjacent rows combined), 4.times.3.75 mm (central 12 rows, joined in groups of three), and 4.times.5.00 mm (all 16 rows, combined in groups of four).
The limitation to only four simultaneously acquired slices in this system is primarily a result of data acquisition hardware limitations, even though it would be possible, for example, to provide as many as 16 slices 1.25 mm thick with the detector hardware.
The limitations of this known CT scanning system are such that large objects, up to 50 cm in diameter, such as a thorax of a patient, are nevertheless scanned rather efficiently. However, for smaller objects, such as a patient's head, utilization of the scanning channels is rather inefficient. A head typically resides in a scan field of view of less than 25 cm. The percentage of detector cells or data acquisition channels contributing to the smaller field of view is only about 56.8%, which means that about 43.2% of the data acquisition channels are idle or providing data not contributing to image reconstruction when the scanner is imaging a patient's head. (The percentages are not exactly 50% because the scanner's detector channels are not concentric with the scanner's rotating gantry.)
It would therefore be desirable to provide methods and apparatus to efficiently utilize the idle or inefficiently used data acquisition channels when imaging smaller objects such as a patient's head.